One day Haru and Kyoko are on their "cake day", a day where they treat themselves while on a diet. They come over to Tsuna's and share cake with I-pin. I-pin shares her Gyoza buns with them. When Haru and Kyoko eat a Gyoza bun that proves fatal to normal people, Tsuna must find I-pin's master who is in town for the day selling buns.